The present invention relates generally to inkjet printing mechanisms, and more particularly to an optical sensing system for determining information about the type of print media entering the printzone (e.g. transparencies, plain paper, premium paper, photographic paper, etc.), so the printing mechanism can automatically tailor the print mode to generate optimal images on the specific type of incoming media without requiring bothersome user intervention.
Inkjet printing mechanisms use cartridges, often called xe2x80x9cpens,xe2x80x9d which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
In closed loop inkjet printing, sensors are used to determine a particular attribute of interest, with the printer then using the sensor signal as an input to adjust the particular attribute. For pen alignment, a sensor may be used to measure the position of ink drops produced from each printhead. The printer then uses this information to adjust the timing of energizing the firing resistors to bring the resulting droplets into alignment. In such a closed loop system, user intervention is no longer required, so ease of use is maximized.
In the past, closed loop inkjet printing systems have been too costly for the home printer market, although they have proved feasible on higher end products. For example, in the DesignJet(copyright) 755 inkjet plotter, and the HP Color Copier 210 machine, both produced by the Hewlett-Packard Company of Palo Alto, Calif., the pens have been aligned using an optical sensor. The DesignJet(copyright) 755 plotter used an optical sensor which may be purchased from the Hewlett-Packard Company of Palo Alto, Calif., as part no. C3195-60002, referred to herein as the xe2x80x9cHP ""002xe2x80x9d sensor. The HP Color Copier 210 machine uses an optical sensor which may be purchased from the Hewlett-Packard Company as part no. C5302-60014, referred to herein as the xe2x80x9cHP ""014xe2x80x9d sensor. The HP ""014 sensor is similar in function to the HP ""002 sensor, but the HP ""014 sensor uses an additional green light emitting diode (LED) and a more product-specific packaging to better fit the design of the HP Color Copier 210 machine. Both of these higher end machines have relatively low production volumes, but their higher market costs justify the addition of these relatively expensive sensors.
In the home printer market, the media may range from a special photo quality glossy paper, down to a brown lunch sack, fabric, or anything in between. To address this media identification problem, a media detect sensor was placed adjacent to the media path through the printer, such as on the media pick pivoting mechanism or on the media input tray. The media detect sensor read an invisible-ink code pre-printed on the printing side of the media. This code enables the printer to compensate for the orientation, size and type of media by adjusting print modes for optimum print quality to compensate for these variances in the media supply, without requiring any customer intervention.
However, media type detection is not present in the majority of inkjet printers on the commercial market today. Most printers use an open-loop process, relying on an operator to select the type of media through the software driver of their computer. Thus there is no assurance that the media actually in the input tray corresponds to the type selected for a particular print request, and unfortunately, printing with an incorrectly selected media often produces poor quality images. Compounding this problem is the fact that most users never change the media type settings at all, and most are not even aware that these settings even exist. Therefore, the typical user always prints with a default setting of the plain paper-normal mode. This is unfortunate because if a user inserts expensive photo media into the printer, the resulting images are substandard when the normal mode rather than a photo mode is selected, leaving the user effectively wasting the expensive photo media. Besides photo media, transparencies also, yield particularly poor image quality when they are printed on in the plain paper-normal mode.
The problem of distinguishing transparencies from paper was addressed in the Hewlett-Packard Company""s DeskJet 2000C Professional Series Color Inkjet Printer, which uses an infrared reflective sensor to determine the presence of transparencies. This system uses the fact the light passes through the transparencies to distinguish them from photo media and plain paper. While this identification system is simple and relatively low cost, it offers limited identification of the varying types of media available to users.
Another sensor system for media type determination used a combination transmissive/reflective sensor. The reflective portion of the sensor had two receptors at differing angles with respect to the surface of the media. By looking at the transmissive detector, a transparency could be detected due to the passage of light through the transparency. The two reflective sensors were used to measure the specular reflectance of the media and the diffuse reflectance of the media, respectively. By analyzing the ratio of these two reflectance values, specific media types were identified. To implement this system, a database was required comprising a look-up table of the reflective ratios which were correlated with the various types of media. Unfortunately, new, non-characterized media was often misidentified, leading to print quality degradation. Finally, one of the worst shortcomings of this system was that several different types of media could generate the same reflectance ratio, yet have totally different print mode classifications.
One proposed system offered what was thought to be an ultimate solution to media type identification. In this system an invisible ink code was printed on the front side of each sheet of the media in a location where it was read by a sensor onboard the printer. This code supplied the printer driver with a wealth of information concerning the media type, manufacturer, orientation and properties. The sensor was low in cost, and the system was very reliable in that it totally unburdened the user from media selection through the driver, and insured that the loaded media was correctly identified. Unfortunately, these pre-printed invisible ink codes became visible when they were printed over. The code was then placed in the media margins to avoid this problem, for instance as discussed in U.S. Pat. No. 5,984,193, assigned to the present assignee, the Hewlett-Packard Company; however market demand is pushing inkjet printers into becoming photo generators. Thus, the margins became undesirable artifacts for photographs with a xe2x80x9cfull-bleedxe2x80x9d printing scheme where the printed image extends all the way to the edge of the paper. Thus, even placing the code in what used to have been a margin when printed over in full-bleed printing mode created severe print defects.
Still another media identification system marked the edge of the media by deforming the leading edge of the media. These edge deformations took the form of edge cuts, punched holes, scallops, etc. to make the leading edge no longer straight, with a straight edge being the plain paper default indicator. Unfortunately these edge deformation schemes required additional media processing steps to make the media. Moreover, a deformed edge lacks consumer appeal, appearing to most consumers as media which was damaged in shipping or handling.
Thus, it would be desirable to provide an optical sensing system for determining information about the type of media entering the printing mechanism, so the printing mechanism can automatically adjust printing for optimal images without requiring user intervention and without damaging the media or the finished image.